The present invention pertains to improvements in the field of storage and transportation of sulfur dioxide. More particularly, the invention relates to a method of confining sulfur dioxide for storage and/or transportation under safe conditions.
Sulfur dioxide (SO2) is a widely used chemical in industries such as wood pulping and bleaching, corn wet milling, water treatment and the production of sulfuric acid. It is a colorless, nonflammable gas with a boiling point of xe2x88x9210xc2x0 C. at atmospheric pressure. Sulfur dioxide is highly toxic by inhalation and a strong irritant to the eyes and mucous membranes. It is also a dangerous air contaminant and constituent of smog.
Currently, bulk quantities of sulfur dioxide are stored and transported as a liquid in suitable pressure vessels. The vapor pressure of liquid sulfur dioxide at temperatures that may occur in normal storage and transport operations can be up to 8 bar. Thus, in case of a leak in or rupture of the pressure vessel used to store or transport liquid sulfur dioxide, particularly if the damage occurs towards the bottom of the vessel, large quantities of sulfur dioxide can be released from the tank very rapidly. Since the sulfur dioxide at ambient temperature is above its boiling point, any liquid sulfur dioxide released to the atmosphere will vaporize rapidly, creating a vapor cloud of toxic gas that tends to stay at ground level, being heavier than air. Prevailing winds can then disperse the vapors, creating conditions hazardous to health or even lethal conditions over a large area. Concentration of 5 to 10 ppmv of sulfur dioxide in air will lead to irritation of the respiratory tract and concentrations above 400 to 500 parts per million by volume (ppmv), even for a few minutes, are dangerous to life. Areas adjacent to industrial SO2 storage sites and railroads or roads used for the transportation of sulfur dioxide are thus at risk in the event of a release.
While it is known that sulfur dioxide dissolves in water to the extent of about 10% by weight, it is not a desirable solvent for the purpose of storing or transporting sulfur dioxide because of the expense of providing large tanks for the dilute solution. Moreover, it is not economical to transport sulfur dioxide in such a water solution because of the excessive cost of transporting nine tons of water for each ton of SO2. Some organic compounds such as chloroform, formic acid, acetic acid, methanol, ethanol and acetone have high solvent power for sulfur dioxide, but these have the disadvantage that they are volatile and would contaminate the regenerated sulfur dioxide with undesirable impurities. In addition, most of these compounds are flammable, thus presenting a fire hazard where none existed before.
Aqueous solutions of alkalis such as sodium hydroxide can dissolve substantial quantities of sulfur dioxide by formation of sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3) and sodium pyrosulfite (Na2S2O5). However, regenerability of sulfur dioxide from these solutions is incomplete, the maximum being about 13% by weight (theoretical) from a saturated aqueous solution of the pyrosulfite. This again implies a very high effective transportation cost for the sulfur dioxide.
It is therefore an object of the present invention to overcome the above drawbacks and to provide a method of confining sulfur dioxide for storage and/or transportation under safe conditions.
In accordance with the present invention, there is thus provided a method of confining sulfur dioxide for storage or transportation under safe conditions, which comprises the steps of:
(a) contacting a sulfur dioxide-containing gas stream with an absorbing medium comprising water and a water-soluble amine absorbent having at least one amine group with a pKa value greater than about 7 and at least one other amine group with a pKa value less than about 6.5 so that the at least one amine group with a pKa value greater than about 7 irreversibly absorbs sulfur dioxide in salt form rendering the amine absorbent non-volatile and the at least one other amine group with a pKa value less than about 6.5 reversibly absorbs sulfur dioxide, to thereby saturate the absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25xc2x0 C.; and
(b) charging the absorbing medium saturated with sulfur dioxide obtained in step (a) into storage or transportation means.
The expression xe2x80x9csafe conditionsxe2x80x9d as used herein refers to conditions presenting a greatly reduced hazard to life and the environment in the case of a leak in or rupture of the storage or transportation container, in comparison to a similar leak or rupture when storing or transporting liquid sulfur dioxide. Since the absorbing medium saturated with sulfur dioxide is below its bubble point, the sulfur dioxide vapor cloud generated by a leak or spill of such a saturated absorbing medium is relatively small. With liquid sulfur dioxide, a very large vapor cloud is formed rapidly since essentially all the sulfur dioxide vaporizes. The use of an amine absorbent having at least one amine group with a pKa value greater than about 7 ensures that the amine absorbent is nonvolatile since such an amine group irreversibly absorbs sulfur dioxide to form a salt which is not regenerable under the normal operating conditions of the process.
Preferably, the amine absorbent has at least one amine group with a pKa value of about 7.5 to about 10 and at least one other amine group with a pKa value of about 4.5 to about 6.0.
Examples of suitable amine absorbents which may be used in accordance with the present invention are diamines having the general formula: 
wherein R1 is an alkylene group having 1 to 3 carbon atoms, R2, R3, R4 and R5 are the same or different and each represent a hydrogen atom, a lower alkyl group having 1 to 8 carbon atoms or a lower hydroxy-alkyl group having 2 to 8 carbon atoms, or any of R2, R3, R4 and R5 form together with the nitrogen atoms to which they are attached a 6-membered ring.
Examples of preferred diamines in free base form include:
N,Nxe2x80x2,Nxe2x80x2-(trimethyl)-N-(2-hydroxyethyl)-ethylenediamine,
N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-ethylenediamine,
N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-diaminomethane,
N,N,Nxe2x80x2,Nxe2x80x2-tetrakis-(2-hydroxyethyl)-ethylenediamine,
N,Nxe2x80x2-dimethylpiperazine,
N,N,Nxe2x80x2,Nxe2x80x2-tetrakis-(2-hydroxyethyl)-1,3-diaminopropane,
Nxe2x80x2,Nxe2x80x2-dimethyl-N,N-bis-(2-hydroxyethyl)-ethylenediamine,
N-methyl Nxe2x80x2-(2-hydroxyethyl)-piperazine,
N-(2-hydroxyethyl)-piperazine,
N,Nxe2x80x2-bis(2-hydroxyethyl)-piperazine,
N-methyl-piperazine, and piperazine.
According to a preferred embodiment, step (a) is carried out in a gas/liquid contact apparatus providing countercurrent gas and liquid flows.
Where the sulfur dioxide-containing gas stream is a gaseous stream of substantially pure water-saturated sulfur dioxide, step (a) is preferably carried out under substantially atmospheric pressure and ambient temperature conditions. On the other hand, when the sulfur dioxide-containing gas stream contains less than about 90% by volume of sulfur dioxide, step (a) is preferably carried out by:
i) contacting the sulfur dioxide-containing gas stream with the absorbing medium to produce a sulfur dioxide-laden absorbing medium;
ii) dividing the sulfur dioxide-laden absorbing medium into separate first and second portions each representing a predetermined proportion of the sulfur dioxide-laden absorbing medium;
iii) removing the absorbed sulfur dioxide from the second portion of sulfur dioxide-laden absorbing medium to regenerate the amine absorbent contained therein and thereby produce a sulfur dioxide-depleted absorbing medium and a gaseous stream of substantially pure water-saturated sulfur dioxide; and
iv) contacting the gaseous stream of substantially pure water-saturated sulfur dioxide with the first portion of sulfur dioxide-laden absorbing medium, whereby the proportion of sulfur dioxide-laden absorbing medium represented by the second portion is such to produce in step (iii) sufficient gaseous sulfur dioxide to saturate in step (iv) the first portion of sulfur dioxide-laden absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25xc2x0 C.
The sulfur dioxide-depleted absorbing medium produced in step (a) (iii) is advantageously recycled in step (a) (i) for absorption of sulfur dioxide. Any sulfur dioxide emissions produced in step (a) (iv) are preferably recycled to step (a) (i) for admixture with the sulfur dioxide-containing gas stream.
When it is desired to use the sulfur dioxide at a consuming site, the absorbing medium saturated with sulfur dioxide is charged into a transport container or pipeline and conveyed to the consuming site where the absorbed sulfur dioxide is removed from the saturated absorbing medium to regenerate the amine absorbent contained therein and thereby produce another sulfur dioxide-depleted absorbing medium and a gaseous stream of substantially pure water-saturated sulfur dioxide for consumption. Preferably, the other sulfur dioxide-depleted absorbing medium is combined with the sulfur dioxide-depleted absorbing medium produced in step (a) (iii) and the combined sulfur dioxide-depleted absorbing media are recycled to step (a) (i) for absorption of sulfur dioxide.
According to another preferred embodiment where the sulfur dioxide-containing gas stream contains less than about 90% by volume of sulfur dioxide, step (a) is carried out by:
i) contacting the sulfur dioxide-containing gas stream with a first absorbing medium comprising water and the amine absorbent to produce a sulfur dioxide-laden absorbing medium;
ii) removing the absorbed sulfur dioxide from the sulfur dioxide-laden absorbing medium to regenerate the amine absorbent contained therein and thereby produce a sulfur dioxide-depleted absorbing medium and a gaseous stream of substantially pure water-saturated sulfur dioxide; and
iii) contacting the gaseous stream of substantially pure water-saturated sulfur dioxide with a second absorbing medium comprising water and the amine absorbent, the first and second absorbing media differing from one another in water content or type of amine absorbent so that the second absorbing medium has an absorption capacity for sulfur dioxide greater than the first absorbing medium, to saturate the second absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25xc2x0 C.
The sulfur dioxide-depleted absorbing medium produced in step (a) (ii) is advantageously recycled in step (a) (i) for absorption of sulfur dioxide. Any sulfur dioxide emissions produced in step (a) (iii) are preferably recycled to step (a) (i) for admixture with the sulfur dioxide containing gas stream.
According to a further preferred embodiment where the sulfur dioxide-containing gas stream contains less than about 90% by volume of sulfur dioxide, step (a) is carried out by:
i) contacting the sulfur dioxide-containing gas stream with a first absorbing medium comprising water and the amine absorbent to produce a first sulfur dioxide-laden absorbing medium and a partially scrubbed sulfur dioxide-containing gas stream;
ii) contacting the partially scrubbed sulfur dioxide-containing gas stream with a second absorbing medium comprising water and the amine absorbent, the first and second absorbing media differing from one another in water content or type of amine so that the second absorbing medium has an absorption capacity for sulfur dioxide less than the first absorbing medium, to produce a second sulfur dioxide-laden absorbing medium;
iii) removing the absorbed sulfur dioxide from the second sulfur dioxide-laden absorbing medium to regenerate the amine absorbent contained therein and thereby produce a sulfur dioxide-depleted absorbing medium and a gaseous stream of substantially pure water-saturated sulfur dioxide; and
iv) contacting the gaseous stream of substantially pure water-saturated sulfur dioxide with the first sulfur dioxide-laden absorbing medium to saturate the first absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25xc2x0 C.
The sulfur dioxide-depleted absorbing medium produced in step (a) (iii) is advantageously recycled to step (a) (ii) for absorption of sulfur dioxide. Any sulfur dioxide emissions produced in step (a) (iv) are preferably recycled to step (a) (i) for admixture with said sulfur dioxide-containing gas stream.
Preferably, steps (a) (i) and (a) (ii) are carried out in a gas/liquid contact apparatus comprising first and second gas/liquid contact zones in gas flow communication with one another. The sulfur dioxide-containing gas stream is contacted in the first zone with the first absorbing medium, the partially scrubbed sulfur dioxide-containing gas flowing from the first zone to the second zone for contact with the second absorbing medium in the second zone. The sulfur dioxide-depleted absorbing medium produced in step (a) (iii) is advantageously recycled to step (a) (ii) for absorption of sulfur dioxide in the second zone. Any sulfur dioxide emissions produced in step (a) (iv) are preferably recycled to step (a) (i) for admixture with the sulfur dioxide-containing gas stream.